In general, multiple input multiple output (MIMO) increases the capacity of a radio link through the use of multiple transmit antennas and multiple receive antennas. MIMO exploits multipath propagation to increase the capacity of the radio link. MIMO has proven to be effective at increasing the capacity of the radio link and has been accepted into a variety of technical standards, including WiFi or Wireless LAN: IEEE 802.11n, and IEEE 802.11ac; Evolved High-Speed Packet Access (HSPA+); Worldwide Interoperability for Microwave Access (WiMAX); and Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) Advanced.
Increasing the number of transmit antennas and receive antennas from a relatively small number (on the order of 10 or fewer) to a significantly larger number (on the order of 100, 1000, 10000, or more) can lead to even greater increases in the capacity of the radio link. Such MIMO communications systems are referred to as large-scale MIMO communications systems.
Channel estimation is a complex and time intensive operation. Under the multi-path model, channel estimation is performed for every multi-path at each antenna and involves individually receiving reference signals transmitted over each multi-path for each transmitter-receiver pair. Therefore, at a MIMO communications device, such as a large scale MIMO communications device, the number of channel estimations can be very large. For example, in a 10000 antenna MIMO communications device with 3 multi-paths, there will be 30000 estimations even for a user equipped with a single antenna.